Tuning control system



P" 7, 1953 R. B. SHOSTAK TUNING CONTROL SYSTEM Filed Feb. 25, 1951 mv wwzoamum Jmzommmm 252; Il l|l| il l l [K mmEjmi ROBERT B. SHOSTAK IN V EN TOR.

Patented Apr. 7, 1953 TUNING CONTROL SYSTEM Robert B. Shostak, Baltimore, Md, assignor to Bendix Aviation Corporation, Towson, Md., a corporation of Delaware Application February 23, 1951, Serial No. 212,445

11 Claims. 1 T

.his invention relates to systems for the contain the output of the balanced modulator untrcl of t e frequency of electrical apparatus and balanced and therefore to cause the tuning momore particularly to a system for the control of tor to maintain a constant high speed in search tuning ilechanism which operates on a frequency until the frequency difference between the two stable electrical device to maintain a constant devices is within prescribed limits and is changfrequency difference between it and an unstable ing in a particular sense. At this point the relay reference device. system restores the modulator to balance and The maintenance of a constant frequency difthe motor will be excited by the output of the ierence between such devices is a dimcult matter, modulator in a manner to track the desired fresince the reference device may depart suddenly l0 quency difference.

unpredictably from its previous frequency, The single figure of the drawing is a schematic necessitatim a search by the tuning mechanism diagram of a circuit embodying the invention. at a speed until the new frequency is found. The circuit illustrated may, by way of example, Once found, the high speed searching action must be utilized for driving a stable local oscillator or be terminated and a slower tracking action stale in a manner to maintain an I. F. of 30 mo. fi i- At h s po t the S se Of t e between its frequency and that of a reference chan fr q n y of e refe ence device with magnetron. A tuning element I of the stalo respect to the stable device must be taken into is driven between limits by a two-phase motor 2 account in the control of the tuning mechanism, havi t field windings 3 and 4 A Source 5 f to insure that the latter drives the stable mech- 2O 60 cycle Voltage supplies exciting voltage of a anism .n the proper direction to reduce the disreference phase which is applied to a transcrepancy, rather than in direction to increase it. former One terminal of the primary of this It is object of this invention to provide a transformer is connected directly to the center control system which causes the tuning cap of th secondary and this junction i grounded mechanism of a frequency stable device to mainand connected to the junction of windings 3 and tain a high c stant tuning speed when the fre-- 2. The other terminal of winding 4 is connected q fiy d' crepahcy between that dev ce and a to the movable contact of a single pole-double reference device exceeds certain limits and, when throw switch. The fixed contacts of this switch Said 8 3 Within Said s, are connected to the terminals of the secondary said tuning mechanism to tune said frequency of transformer 6. The switch I is actuated by stable de ice in the pr p e se n l $0 the motor 2 when the tuning element I is driven as to a1 -v tand to maintain constant a desired to either of its limits, It can be seen that acfr q na ff ren w n said v tuation of the switch 7 reverses the phase of the It is a further Object 0 the invention to p voltage across winding 5. The terminals of the r de $11811 a 537556111 Which s l9le of ccnstrucwinding 3 are connected across the secondary of tion and reliable in operation. a transformer 55. The derivation of the voltage The above and other ObJ' tS d ad n s applied to this transformer will be explained later. 'of the inventi e realized y a System in Which The discriminator is not illustrated in detail a element f h frequency stable device in the drawing since it is of conventional form, is driven between limits at a high fixed speed in 40 b t i i di t d as a b [(3, By ay of example, Search. Th d v 61611181113 is a two-phase O- it may be assumed, as previously stated, that it is 3 this phage Of 0118 flilmpohent OI" the exciting desired to maintain a receiver intermediate fre- Veltfige hell-1g mechanically reversed When the quency at 39 megacycles with respect to the fretu l l s re ad ed. A discriminatfll p quency of some unstable reference source such as vides opposed D. 0. error voltages, the response amagnetron. In this event the discriminator may e p aked at points equidistant from a be constructed to have a peaked. response at 29 sired frequency difference. The discriminator d t 31 me. It will then have D. C. error outp ts are applied to a balanced modulator to 15 g9 t uts proportional to the proximity of which i also ppl a v s v n the same the i. frequency to these 1" equencies. The outeS ce Voltage compo the put due to 31 mo. response is indicated as applied tuning motor exciting voltage. The output or" the b l d ii d that due to 29 mo. response as balanced modulator is utilized as the second com- 9,13 d by lead it to the end terminal of an antipcnent the tuning motor exciting voltage. hunt circuit 52-.

A relay system provided which responds to T circuit comprises a voltage divider com the discriminator output in a manner to maim 55 posed of four serially connected resistors it, 15,

I5 and i1. Resistors l4 and I! are shunted by condensers l8 and i9 respectively. The complete circuit is balanced above ground and may be said to have a floating ground. This circuit is necessary in order to avoid over-shoot of the tuning motor as the I. F. approaches 30 mo. At this part of the discriminator characteristic curve, the error voltage is changing most rapidly as indicated by the fact that the typical discriminator characteristic curve has its greatest slope in this region. The time constants of these R. C. circuits at the ends of the voltage divider are made such that if the error voltage is changing fast enough to cause over-shoot, the condensers will not fully discharge, the voltage due to the charge remaining on the condensers will be added to the error voltage, and the balanced modulator will see an opposite polarity before the true crossover point is reached. The tuning motor will therefore be slowed down and will coast into final pos tion. locking in at the 30 mc. I. F. point.

The balanced modulator is indicated at 20. It is composed of two triodes 2| and 22. The grid of tube 2| is connected through a resistor 23 to the junction point of resistors I4 and I5 and the grid of tube 22 is connected through resistor 26 to the junction of resistors l6 and H. The cathode of tube 2| is connected to the electrical center of the voltage divider through resistor 26 and the cathode of tube 22 is connected to the same point through a variable resistor 25. The latter resistor is a centering control for compensating for differences in the anti-hunt divider and the two sections of the balanced modulator. The resistors 23 and 25 are for the purpose of limiting grid current flow. The anodes of tubes 2| and 22 are connected to the opposite terminal of a parallel resonant circuit comprising the primary winding of the transformer 30 shunted by a condenser 3|. This circuit is tuned to the frequency of the tuning motor exciting current. A relay actuated switch 32 interrupts one anode circuit. This switch is closed when the balanced modulator is functioning as such.

Voltage from the source 5 is applied to the balanced modulator through a transformer 34, the secondary of which is connected between the junction point of resistors l5 and I6, and the center tap of the primary of transformer 38. The secondary of the transformer 30 is connected to a servo amplifier 54 the output of which is applied to the tuned primary of transformer 53.

In operation, the balanced modulator conducts only during the half of each cycle when the plate circuit is positive with respect to the cathode. When there is no error voltage applied to the modulator both tubes conduct equally and, since the outputs at the fundamental frequency of source 5 are 180 out of phase, they cancel in the transformer 30. If there is an error voltage applied to the modulator, one half of it conducts more than the other according to the polarity of the resultant current fiow through the voltage divider 3, which in turn depends upon the direction of frequency error. There is, therefore. a greater flow of current through one half of the primary of transformer 30 than through the other. There is a voltage induced into the secondary of transformer 30, due to this resultant flow of current in the primary, in direct proportion to the magnitude of the error voltage, and of one of two opposite phases, the phase depending upon which half of the primary is conducting the greater current flow. The output of the balanced modulator is thus a modulated 60 cycle voltage which is amplified and utilized as one component of the exciting voltage of the tuning motor.

When the relay operated switch 32 is open one half of the balanced modulator is rendered inoperative and the remaining half, including the tube 22 and the lower half of the primary of transformer 30, acts merely as a tuned coupling, causing a fixed phase cycle voltage to be induced in the secondary winding and the tuning motor to be driven at rated speed in search, regardless of the discriminator output.

Control of the operation of the switch 32 is provided by a relay system which will now be described.

The system includes a pair of cathode coupled triodes 40, 4|. The component of the discriminator error voltage due to 29 mo. response is applied through leads |2 and 42 and resistor 43 to the grid of tube 49. The tube 4| is normally rendered conductive by the application of a fixed positive voltage by way of a potentiometer 42 to its grid. The plate lead of this tube includes the coil of a relay 45 operating a switch 46. The switch is shown in closed position which is the energized position of the relay.

The contacts of this switch are connected to respective contacts of a switch 41. The lower contacts of switches 46 and 4! are connected to a source of plate supply voltage. The movable contact of switch 46 and the upper contact of switch 37 are connected to the plate circuit of a triode t8. This tube is cathode coupled to a triode 49. The plate of tube 49 is connected to the movable contact of switch 41. The component of the discriminator error voltage due to the 29 mo. response is applied to the grid of tube Q8 through resistor 50. The component of discriminator error voltage due to 31 mc. response is applied to the grid of tube 49 through a resistor 5|. In the plate circuit of tube 48 is the coil of a relay 52 which operates switches 41 and 32. These switches are shown in the positions they occupy when the relay 52 is deenergized.

In the drawing the system is shown with the various switches in the positions they occupy when the I. F. is far from the desired 30 mc. value. The switch 32 is open and the tuning motor is consequently turning at rated speed, searching for the frequency of the magnetron. The tube ii is receiving plate supply voltage through switch 41. The switch 4'! is short circuited by switch 45. Relay 45 is energized and will remain so until the I. F. approaches the vicinity of 29 me. When this happens, and the voltage on the grid of tube 4% is raised sufiiciently, the tube conducts. Due to the cathode coupling the tube 4| becomes nonconductive and the relay 45 is deenergized. The relay will remain deenergized while the I. F. is within a region extending from below 29 mo. to well above 30 mc. When this frequency goes beyond 31 mo. the relay will again be energized.

When the relay 45 is deenergized, however, the switch 46 will be opened and the short circuit of switch 47 removed. Due to the direct cathode coupling of tubes 48 and 49, however, the relay 52 will not be actuated until the discriminator 29 mo. response exceeds the 31 mc. response. When this occurs tube 48 will conduct and switch ll? will be actuated, closing its movable and upper contacts. This will cause the output of tube 49 to be applied, together with that of tube 28, to the relay 52 and will maintain it in an energized state regardless of which component of the discriminator error voltage subsequently becomes the greater, The energization of relay 52 closes switch 32 so that the modulator 20 becomes balanced and its output causes the tuning action to be one of tracking.

Should the reference frequency change so that the I. F. departs substantially from the vicinity of the 29 mo. frequency the relay 45 will again be energized, the switch 41 will be shorted by the closing of switch 46 and the relay 52 will become deenergized. The tuning device will thereupon begin searching again.

The effect of the above described mode of operation is to prevent the system from going into its tracking function unless the I. F. is changing toward 30 me. from a value which is less than that. It is necessary that a restriction of this type be incorporated into the system in order to prevent the possibility of the tuning element being driven in a direction to increase, rather than diminish, the error.

In order to make more clear the necessity for this restriction, let us consider two possible sets of circumstances. Let it first be assumed that the magnetron frequency is at 1290 me. and the stable local oscillator, or stalo, is at 1310 me. and is being caused to search toward 1320 inc. The switches will be in the positions illustrated with relay 45 energized and relay 52 deenergized. The modulator 29 will be unbalanced, with the output of tube 22 alone providing the voltage which determines the direction of tuning motor rotation. As the stale approaches 1319 me. (29 Inc. I. F.) relay 45 deenergizes and then relay 52 is energized.

With the closing of switch 32 the modulator it becomes balanced and the tuning motor will thence forth be driven in track. But, since the 29 mo. discriminator response will still predominate, the phase of the modulator output will continue to be dictated by the predominance of current fiow through tube 22 and will therefore remain unchanged. The tuning motor will continue to be driven in the same direction until the 30 mo. 1. F. is reached, whereupon the stalo will be locked in.

Now let it be assumed that the magnetron is at 1290 me. and the stalo is at 1258 me. and being driven toward 1260. In other words, the I. F. is approaching 30 me. from a high to a low value. The switches will again be as shown. When the stalo approaches 1259 mo. (31 Inc. I. F.) relay 45 will be deenergized. But the relay 52 will not be energized until the I. F. has gone on past 30 mo. and approaches 29 mo. Again the phase of the output of the balanced modulator will be unchanged as it goes from the search to the tracking output and the tuning motor will continue tuning in the same direction until the I. F. is so low that relay 45 again becomes energized. The tuning motor will drive until the tuning limit is reached and then reverse. As the I. F. now increases the system will lock in on 30 me. as in the first example. Alternatively, if the 30 mo. I. F. is found on the other side of the magnetron frequency before the limit is reached, it will lock in.

Let us suppose now that in the example just given, with the I. F. decreasing toward 30 inc. the relay 52 had been actuated at 31 me. just after relay 45 was denergized. With the closing of switch 32, the current flow through tube 2! would predominate over that through tube 22 and the phase of the output of modulator 29 would be reversed with respect to the phase it had had during the searching condition just preceding. The direction of rotation of the tuning motor would be reversed and the stalo frequency would start decreasing while the I. would be increasing toward 32 inc. As the tuning motor reached the end of its travel and was automatically reversed the same set of conditions would again come into being and with the same result. An I, F. of 30 me. could not be reached under such circumstances, but would simply be approached as a limit, so hat the stalo would be continuously tuned to and fro between this limit and that provided by the reversing switch.

What is claimed is:

1. A system for maintaining desired fixed frequency separation between a tunable alternating current generator and an unstable reference source of alternating current, said system comprising means for tuning said generator back and forth between fixed frequency limits, a discriminator having response peaks at two frequencies above and below said separation frequency, means applying to said discriminator a voltage having a frequency which is the difference between the frequencies of said generator and said source, and means responsive to the outputs of said discriminator to control said tuning means, said control means comprising a balanced modulator having the outputs of said discriminator applied thereto, means applying the output of said balance-d modulator to said tuning means in a manner to co direction of tuning in accordance with the phase thereof and the speed of said tuning accordance with the amplitude thereof. means operable to disable one-half of said balanced modulator, and means for the control of said disabling means. the last named control means comprising a first relay operable when energized to actuate said disabling means, means responsive to a preponderance of the out put of said discriminator at one end of said peak freouencies to energize said relay, disabling means for said first relay, a second relay operative when enei, ed to actuate the last named disabling and means energizing said second relay, the named means being responsive to a predetermined level of response of said discriminator at said one of said peak frequencies to denergize said second relay.

2. A system for i t aining a desired fixed fre quency separation between a tunable alternat' .g current genera or and an unstable reference source of alternat n current, said system comprising means i" ing said generator back and forth between fi ed frequency limits, disc-1 inator ineans ive to the difference 1' quencr between t puts oi generator and said source h response peaks at a air of frequencies above and below said desired separation frequency, me controlling said tuning means and having ..0 operating states, in the first of which it Jrives said tuning means at a fixed ra he an limits, and in the second of which it drlv 5 said tuning means in a direction which is a func ion of the sense of the difference of the out uts of said discriminator at said pair of es and at a rate which is a function the "llL-J said difference, and means selecting the operating states of said controlling rneans a function of the value and direction of change of said difference frequency, said selecting means comprising a first relay controlled by the output of said discriminator and operable to estal 11 said first state when the output of said discriminator at one of said peaked response points predominates, a second relay op-v erable when energized to disable said first relay and means operable when the output of said discriminator at said one of said peaked response points reaches a predetermined level to deenergize said second relay.

3. A control system responsive to the frequency of an alternating current to actuate a control element, said system comprising a discriminator having a pair of outputs, the amplitude of each of said outputs being a function of the proximity of the frequency of said alternating current to a respective reference value, a first relay operable when energized to actuate said element, means responsive to the predomination of one of said outputs to energize said relay, means normally maintaining said second relay in an energized state, a second relay operable when energized to disable said first relay and means operable when said one of said outputs of said discriminator reaches a predetermined level to deenergize said second relay.

4. A control system responsive to the frequency of an alternating current to actuate a control element, said system comprising a discriminator having a pair of outputs, the amplitude of each of said outputs being a function of the proximity of the frequency of said alternating current to a respective reference value, a relay operable when energized to actuate said element, means for energizing said relay when the amplitude of one of said outputs is greater than the amplitude of the other, said energizing means comprising a pair of electron discharge devices each having a cathode, an anode and a control electrode, the cathodes of said devices being directly coupled, means applying each of said outputs to a respective one of said control grids, a source of plate supply voltage, the coil of said relay being connected between said source and the plate of the one of said devices to the grid of which said one of said outputs is applied, a switch actuated by said relay and operable when said relay is energized to connect the plate of the other of said devices to said source through said relay coil and operable when said relay is deenergized to establish an alternative connection between said plate of said other device and said source, and means operable when said one of said outputs reaches a predetermined level to short circuit said switch.

5. A control system as claimed in claim i, the last named means comprising a second relay, a second switch actuated by said second relay and operable when said second relay is energized to short circuit the first mentioned switch, and means responsive to said one of said outputs and operable when said output exceeds a certain level to energize said second relay.

6. A control system as claimed in claim 4, the last named means comprising a second relay, a second switch actuated by said second relay and operable when said second relay is energized to short circuit the first mentioned switch, and means responsive to said one of said outputs and operable when said output exceeds a certain level to energize said second relay, the last named means comprising a pair of electron discharge devices, each having a plate, a cathode and a control electrode, the cathodes of said devices being directly connected, the first of said devices having the coil of said second relay connected in its plate circuit and a fixed bias voltage applied to its control electrode whereby said second relay is normally energized, and means applying said one of said discriminator outputs to the control electrode of the second of said devices,

the value of said fixed bias voltage being so selected that said second device will be rendered conductive and said first device non-conductive when said one of said outputs reaches a certain value.

7. A system for maintaining a desired fixed frequency separation between a tunable alternating current generator and an unstable source of alternating current, said system comprising means for tuning said generator back and forth between fixed frequency limits, discriminator means responsive to the difference frequency between the outputs of said generator and said source and having response peaks at a pair of frequencies above and below said desired separation frequency, means differentially comparing the outputs of said discriminator at said response peaks and deriving from said comparison an alternating control voltage having a phase and magnitude corresponding to the sense and magnitude of the difference between said discriminator outputs, means applying said control voltage to said tuning means in a manner to control the direction and rate of tuning as a function of the phase and magnitude of said control voltage, a switch for interrupting, when open, the application of one of said discriminator outputs to said comparin means, and operating means for said switch comprising a pair of'relays, the first of said relays being connected to close said switch when energized, means responsive to the outputs of said discriminator and operable to energize said first relay when the amplitude of the other of said discriminator outputs is greater than that of said one of said outputs, means connecting the second of said relays to disable said first relay when it is energized and means operable to deenergize said second relay when said other of said discriminator outputs exceeds a certain value.

8. A system for maintaining a desired fixed frequency separation between a tunable alternating current generator and an unstable source of alternating current, said system comprising means for tuning said generator back and forth between fixed frequency limits, discriminator means responsive to the difference frequency between the outputs of said generator and said source and having response peaks at a pair of frequencies above and below said desired separa tion frequency, means difierentially comparing the outputs of said discriminator at said response peaks and deriving from said comparison an alternating control voltage having a phase and magnitude corresponding to the sense and magnitude of the difference between said discriminator outputs, means applying said control voltage to said tuning means in a manner to control the direction and rate of tuning as a function of the phase and magnitude of said control voltage, and means operable to interrupt the application of one of said discriminator output voltages to said comparison means, the last named means being responsive to said discriminator outputs and operable to interrupt said application except when said difference frequency is approaching the value of said desired frequency separation in a predetermined sense, said last named means comprising a switch and a pair of relays, the first of said relays being operable when energized to drive said switch to its non-interrupting position, means responsive to the outputs of said discriminator and operable to energize said first relay when the amplitude of the other of said discriminator outputs is greater than that of said one of said outputs, means connecting the secnd of said relays to disable said first relay when it is energized and means operable to deenergize said second relay when said other of said discriminator outputs exceeds a certain value.

9. A system for maintaining a desired fixed frequency separation between a tunable alternating current generator and an unstable source of alternating current, said system comprising means for tuning said generator back and forth between fixed frequency limits, discriminator means responsive to the difference frequency between the outputs of said generator and said source and having response peaks at a pair of frequencies above and below said desired separation frequency, means differentially comparing the outputs of said discriminator at said response peaks and deriving from said comparison an alternating control voltage having a phase and magnitude corresponding to the sense and magnitude of the difference between said discriminator outputs, means applying said control voltage to said tuning means in a manner to control the direction and rate of tuning as a function of the phase and magnitude of said control avoltage, and means operable to interrupt the application of one of said discriminator output voltages to said comparison means, the last named means being responsive to said discriminator outputs and operable to interrupt said application except when said difference frequency is approaching the value of said desired frequency separation in a predetermined sense, said last named means comprising a switch, means operable when the amplitude of the other of said discriminator outputs is greater than that of said one of said outputs to drive said switch to its non interrupting position, and means responsive to said other of said discriminator outputs and operable to disable said driving means except when the amplitude of said other output exceeds a certain value.

10. A control system responsive to the frequency of an alternating current to actuate a control element, said system comprising a discriminator having a pair of outputs, the amplitude of each of said outputs being a function of the proximity of the frequency of said alternating current to a respective reference value, a relay, means connecting said relay to actuate said control element when said relay is energized, switch means connected to energize said relay, said switch means having applied thereto the outputs of said discriminator and being 0perative to energize said relay when a selected one of said outputs exceeds the other, means connected to said relay to normally disable the same, and means applying said one of said outputs to said disabling means, said disabling means being responsive to a predetermined level of said output voltage to terminate its disabling action.

11. A control system responsive to the frequency of an alternating current to actuate a control element, said system comprising a discriminator having a pair of outputs, the amplitude of each of said outputs being a function of the proximity of the frequency of said alternating current to a respective reference value, a relay, means connecting said relay to actuate said control element when said relay is energized, said switch means comprising a pair of electron discharge devices each having a cathode, an anode and a control electrode, means coupling said cathodes, means coupling a respective one of said outputs to each of said control electrodes, means connecting said relay to the anode of the one of said devices to which a selected one of said outputs is coupled, means connecting the anode of the other of said devices to said relay, the last named means comprising a switch actuated by ,said relay, a source of plate supply voltage, said ,switch when in one position connecting said anode to said relay and when in another position disconnecting said anode from said relay and :connecting it to said source, disabling means for said relay, and means rendering said disabling means inoperative when said selected one of said outputs exceeds a predetermined level.

ROBERT B. SHOSTAK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,369,542 Dietrich Feb. 13, 1945 2,478,977 Nicholson Aug. 16, 1949 

